It is known to measure fluid flow using thermal time of flight. In a thermal time of flight measurement the fluid is heated locally. Subsequently, the temperature is measured at different positions to detect how fast a locally heated fluid element is transported by flow of the fluid. In the known measurement the fluid flows through a conduit and temperature sensors attached to the wall of the conduit, or inside the conduit, are used to measure temperature of fluid that is in contact with the sensors. This method of measuring has the disadvantage that the measurement result partly depends on the relation between conduit properties and fluid properties other than flow speed. With decreasing conduit size and flow speed these other dependences increasingly affect the accuracy of the measurement.
Another known fluid flow measuring technique is ultrasonic time of flight measurement. In this technique measurements are made of the time intervals needed to pass ultrasound a certain distance against the flow and along the flow respectively. Flow speed can be computed from the difference between the measured time intervals and the distance travelled by the sound. Due to limitations on time measurement this technique is only accurate for sufficiently high flow speed.
In PCT patent application No PCT 2007/NL050550, which was unpublished at the time of filing the present patent application it is described that fluid flow may be measured by heating the fluid with a laser with a heating pattern that sinusoidally depends on time and detecting the time of flight from downstream measurements of sound speed in the fluid.